You know what I'd love to see instead of a spin-off film? As series. With Marvel really starting to explore the possibilities of TV/Netflix with different types of heroes and flashback series, I think this would be great. In fact, I'm jealous of Daredevil fans who will get 13 hours to explore his story, while we only have a few films to explore what makes Thor tick. This would be like a "Thor origin" of sorts. I've mentioned elsewhere my ideas for a series set in Asgard between Thor 2 and Thor 3, but with Agent Carter's series successfully fleshing out some MCU history, here's another idea I've had:

Son of Asgard

Spoiler!!! Click to Read!:

Set roughly 1,000 years ago (a bit after the opening sequence of the first Thor movie), Thor, Loki, Sif, and the Warriors Three are teenagers growing up in Asgard. Thor has yet to prove himself worthy of Mjolnir, and Loki is still an ally.

The first episode starts with Thor in Odin's Vault, trying and failing to budge Mjolnir. He is brash and overconfident. Things come easily to him (other than lifting Mjolnir), but despite his headstrong nature, he is still noble and good-natured, charming and likable. Wanting to prove himself worthy, Thor decides to set out on his own to retrieve the powerful Warlock's Eye from Harokin. However, his brother hears his plan and informs Thor's friends. Instead of stopping him like Loki hoped, they think it's a great idea and join him!

The young heroes are tested along the way, as they encounter the dangers of the Asgardian countryside for the first time, having never left the golden city. Thor and his companions are all given the chance to show their unique skills (Loki's intelligence, Sif's boldness, etc.) and they all get far more time to shine than they ever would in the films. Fandral, Hogun, and Volstagg go on a side mission and really cement their strong bond as an inseparable trio. Thor even develops his erratic control over the storm, realizing how powerful he is but how uncontrollable it is--he needs some way to focus it. In the series finale, Sif and the Warrior's Three are captured and in mortal danger, but Loki devises a way for Thor to use his weather control to save them.

By the end of the series, the heroes return to Asgard with the Warlock's Eye, but rather than being treated as heroes, they are publicly chastised. In private, however, Odin commends Thor for saving the lives of his companions. Thor is angry to find that he still cannot lift Mjolnir, although he has learned some lessons about being a leader, and has gotten closer to lifting Mjolnir. Loki has grown more resentful of Thor than ever. Sif has grown into a self-confident warrior, and the Warriors Three have become inseparable. If there were more than one season, eventually Thor would lift Mjolnir and learn how to use it to channel his innate powers.

As a Thor fan, I would love to get to explore the character in more depth, but I know there is no way that the stars of the film would ever do a series. This would be a practical way to get a series with Thor, as younger actors would be cast for all of the characters (with the exception of Odin, Heimdall, and/or Frigga, who could be worked around somehow). Most of the series would take place outside of the City of Asgard, in forests or castle-like sets, so the FX-heavy settings would be reduced. The series could go on Netflix (look at the costumes and setting of the Marco Polo Netflix series for an idea of how this show could look).

The series would explore the background and mythology of the 9 Realms, and could even add some connectivity with mentions of the Kree empire, Infinity Gems, Celestials, or what-have-you. Lorelei, Amora, even Eliot Randolph or the Berserker Staff (from SHIELD) could appear or be be referenced. The story's focus, however, would be on developing the relationship between Thor and his allies. The young warriors could possibly have older Asgardians as mentors, or they could be in a "warrior-school" like in the Son of Asgard comic book that inspires this series. There have been some references in the Thor movies that could be explored here (like the Battle of Harokin that was mentioned in TDW, or the young warriors obtaining some of the items in Odin's Vault). Multiple seasons could even see Thor traveling to Midgard.

Inspirations: Son of Asgard comic, the Thor: Blood Oath quest story, the numerous Tales of Asgard stories of a young Thor from early Thor comics, and the younger Thor from Jason Aaron's run (Thor could even use Jarnbjorn since he doesn't have Mjolnir yet).

However, I hesitate to suggest this because I wasn't a huge fan of the Tales of Asgard animated movie that featured a young Thor and Loki. It was okay, but not great. I also don't like Smallville-type stories, and I know that's what this sounds like. However, I prefer a Thor with Mjolnir (just like I prefer heroes in costume, etc), and I would want the young Thor of this series to get it and use it before it's end!

^ I remember it being mentioned in an interview awhile back, with a sort of "anything's possible" or "we've thrown lots of ideas around" kind of vibe. But if I remember correctly, years have passed and the MCU has changed quite a bit since that interview. Who knows, though, anything could happen!